This invention is related to a lens apparatus. More specifically, it is related to a lens apparatus which improves a lens supporting condition of photographing cameras, video cameras, and the like.
A conventional lens unit and its lens barrel are shown in FIG. 4 as an example of conventional lens systems.
In FIG. 4, 11 is a lens barrel, the material of which is a policarbonate resin being filled with glass fibers for strengthening the material (hereinafter this material is referred as GFPC: glass filled policarbonate). 1 is a glass lens. 2 is a plastic lens, the material of which is a policarbonate resin (hereinafter this material is referred as PC: policabonate). 3 is a plastic lens, the material of which is an acrylic resin (hereinafter this material is referred as PMMA: polymethyl metacrylate).
The three of lenses 1, 2 and 3 are respectively coupled with the lens barrel 11 as shown in the figure. The lenses are supported by a lens supporter, which is not shown in the figure, so that the lenses are prevented from being displaced from the right side of the barrel and the three of the lenses are constantly urged to the optical axis direction by a force of a suitable strength in a direction to the left side.
As the size of an apparatus, such as a camera, reduces recently, the influence on the focal plane of a lens system (so called "error sensitivity"), caused by discrepancies of the axes of the lenses from the optical axis of the lens system, becomes great. In the present example of a lens system, if one of the lenses has a discrepancy of 5 .mu.m from the optical axis by shifting, an image deterioration caused by the discrepancy from the optical axis makes the normal function of the lens system impossible. Therefore, the difference between the outer diameter of a lens and the inner diameter of the lens barrel (hereinafter it is referred as "a clearance") always needs to be within the range of 0 .mu.m-10 .mu.m.
However, each lens of a lens system creates a different dimensional change according to a respective characteristic of the material when the lens system is placed in the condition of high temperature and humidity. For example, when a lens of the present example, having an outer diameter of about 10 mm, is moved from the condition of normal temperature and humidity to the condition of 60.degree. C. in temperature and 90% in humidity, the change of the inner diameter of the lens barrel 11, which is made of a GFPC, is an increase between 0 .mu.m and 2 .mu.m because of the effects of glass fibers. In the same condition, the changes of the outer diameters of the glass lens 1 and the plastic lens 2, both being made of a PC, and of the plastic lens 3, being made of a PMMA, are respectively almost nothing, an increase of 5 .mu.m, and an increase of 26 .mu.m.
Therefore, if the clearances between the respective lenses and the lens barrel under the normal temperature are set to 8 .mu.m, the clearances for the glass lens 1 and the plastic lens 2, being made of a PC, respectively become between 8 .mu.m and 10 m and between 3 .mu.m and 5 .mu.m in the above condition; however, the clearance for the plastic lens 3, being made of a PMMA, becomes between -16 .mu.m and -18 .mu.m in which the outer diameter of the lens exceeds the inner diameter of the barrel and the lens is tightly caulked by the barrel.
A plastic lens is easily distorted by an external force; therefore when it is in above condition, the plastic lens 3 comes into the circumstance that a pressure is applied from its circumference, the curved surfaces of the lens are seriously distorted, and an image through the lens is deteriorated. However, if the clearance of 18 .mu.m is set in a normal temperature, the optical axis of the lens can unpreferably be displaced. Further, additives such as glass fibers, which can avoid the influences of temperature, humidity and an external force, can not be mixed in a plastic in use with an optical system. Therefore, when plural lenses, which are made of different materials, are configured in one lens barrel, there are the serious problems explained above.